The present invention relates to a method of magnetic refrigeration and a magnetic refrigerating apparatus utilizing the magnetocaloric effect.
Magnetic refrigerating apparatuses of this type are used to condense helium or other condensable gases, based on heat absorption by a magnetic substance, cooled by adiabatic demagnetization. These apparatuses have an advantage over conventional compression-type refrigerators in having a greater refrigerating capacity per unit volume.
Referring now to an entropy diagram of FIG. 1, the refrigeration cycle of one such prior art magnetic refrigerating apparatus will be described.
In FIG. 1, the axes of ordinate and abscissa represent entropy and absolute temperature, respectively. If a working substance or magnetic substance, such as gadolinium-gallium-garnet, is rapidly introduced into a magnetic field, its temperature rises from level Tc to Tm (from point C to D in the graph; adiabatic magnetization process). If heat, then produced in the working substance, is expelled to the outside, the entropy lowers (from point D to A; isothermal magnetization process). Then, if the working substance is quickly taken out of the magnetic field, its temperature drops from level Tm to Tc (from point A to B; adiabatic demagnetization process). If temperature Tc is equal to the condensation temperature of the condensable gas, the working substance condenses the gas by absorbing heat therefrom, so that the entropy increases from point B to C (isothermal demagnetization process).
In general, the refrigerating capability or capacity of refrigerators is determined by the difference between exo- and endothermic temperatures and the level of heat absorption. In the refrigeration cycle described above, however, if the temperature difference (Tm-Tc) between heat-discharge (high-temperature) side and heat-absorption (low-temperature) side becomes greater, the difference in entropy, i.e., the amount of absorbed heat, lowers from point AA, through BB and C, to DD of FIG. 1. Since this situation is attributable to the physical properties of the working substance, it has hitherto been considered difficult to fulfill these two conflicting requirements at the same time.